1. Field of the Invention
The present invention relates to a continuous process for preparing a spinning solution of acrylic polymers. More particularly, the invention relates to a continuous process for preparing a spinning solution of acrylic polymers produced by bulk polymerization, and utlizing as the solution solvent, an organic compound such as dimethyl acetamide, dimethyl formamide, ethylene carbonate, dimethyl sulfoxide, etc.
"Acrylic polymers" as that term is used herein, is intended to mean long-chain synthetic acrylonitrile homopolymers or copolymers containing 85% or more by weight of units derived from acrylonitrile and having a numerical molecular weight of at least 20,000 but preferably not in excess of 60,000. If an acrylonitrile copolymer is employed, then the remainder comprises units derived from at least one other ethylenically unsaturated monomer that is copolymerizable with acrylonitrile.
2. Description of the Prior Art
As is well known, acrylic polymers may be prepared by suspension polymerization, solution polymerization, or bulk polymerization.
Polymerization processes conducted in an aqueous suspension offer the advantages of a relatively high polymerization rate and that of providing a resulting polymeric material which exhibits desirable qualities, such as, e.g., a high degree of purity, whiteness, etc.
However, the necessity of obtaining the polymer in the dry state so as to utilize it in a spinning step with organic solvents, requires complex, tedious, and expensive operations, such as filtration, drying and grinding. The dried polymer is dissolved in the spinning solvent by successive incremental addition of the polymer under stirring.
When polymerization is carried out in solution, it is necessary to operate with a highly viscous medium in the polymerization reactor. The high viscosity of the medium limits the heat exchange co-efficient and, as a result, the reactor dimensions. Moreover, since the polymeric solution is very viscous, separation of that portion of the monomer which is not converted into polymer must be carried out in special, complex, complicated apparatus.
As is well known, in the bulk polymerization of acrylonitrile one operates in the absence of both water and solvent. According to this technique, and where the polymer is to be employed for producing fibers, films, and other shaped articles, the reaction mass leaving the reactor must be subjected to a mechanical separation of polymer from monomer (centrifuging and/or filtration), and the polymer is thereafter dried, ground and dissolved in the solvent.